This invention relates to a new process for the preparation of polyhydric alcohol 3-alkylthiopropionate ester stabilizers for polyolefin resins. Polyhydric alcohol 3-alkylthiopropionate ester stabilizers have been disclosed previously, but their preparation conventionally has required the use of relatively costly starting materials and furnished products that when used to stabilize polyolefin resins have contributed to the formation of undesirable yellowish and beige discolorations as well as unpleasant odors when the stabilized resins were processed or heat aged at elevated temperatures.
Thioether carboxylic acid esters and their use for stabilizing polymers have been discussed by M. Minagawa et al, in U.S. Pat. No. 4,106,629 of Aug. 8, 1978, and for the sake of brevity it is sufficient here to refer to Minagawa's discussion of this art.
Trihydric and higher valent polyhydric alcohol 3-alkylthiopropionate ester stabilizers have conventionally been prepared as disclosed, for example, by M. Dexter in U.S. Pat. No. 3,758,549 of Sept. 11, 1973 by reaction of a polyhydric alcohol such as pentaerythritol with methyl 3-laurylthiopropionate by transesterification and subsequent purification of the product, such as by chromatography on a column of alumina. Dexter disclosed the preparation of the required 3-alkylthiopropionic acid ester intermediate by reaction of the alkyl mercaptan with methyl acrylate and purification of the intermediate by high vacuum distillation, so that Dexter's process comprises two synthetic steps ad two purification steps that present considerable difficulty in commercial scale operation.
M. Minagawa in Japanese Kokai No. 75/106881 of Aug. 27, 1975 disclosed stabilized resin compositions containing 3-alkylthiopropionate esters of alcohols containing a nitrogen-heterocyclic ring, for example tris(2-hydroxyethyl isocyanurate) and optionally a phenolic antioxidant.
Minagawa's esters are shown as synthesized by esterification of the nitrogen-heterocyclic alcohol with a 3-alkylthiopropionic acid, without a suggestion of how the required 3-alkylthiopropionic acid intermediates were to be obtained.
Disclosures of the preparation of 3-alkylthiopropionic acids, their lower alkyl esters, and their metal salts include M. Gribbins U.S. Pat. No. 2,416,052 of Feb. 15, 1947, who added mercaptans to acrylonitrile followed by acid or alkaline hydrolysis of the nitrile group; W. Leistner in U.S. Pat. Nos. 2,680,107 of June 1, 1954 and 2,723,965 of Nov. 15, 1955, who added alkyl mercaptan to methyl acrylate with sodium methylate catalyst, hydrolyzed the resulting methyl ester to the sodium salt of the acid by heating with NaOH, and then acidified to isolate the acid; and I. Yamane in U.S. Pat. No. 3,741,909 of June 26, 1973 who disclosed stable organic material prepared by mixing with a compound expressed by the general formula; ##STR1## (wherein R represents hydrocarbon radical having 6-22 carbon atoms, X represents a lower alkylene radical having 1-5 carbon atoms, M represents non-alkali metal and n represents an integer ranging from 1 to 5) along with a phenol-type antioxidant or amine-type antioxidant.
This compound is synthesized through the following process; ##STR2##